Progressive cavity pumps are used for artificial oil lifting operations on wellheads. FIG. 1 illustrates a typical progressive cavity pump system 10 for a wellhead 12. The progressing cavity pump system 10 has a surface drive 20, a drive shaft 30, and a downhole progressive cavity pump 40. At the surface of the well, the surface drive 20 has a drive head 22 mounted above wellhead 12 and has an electric or hydraulic motor 24 coupled to the drive head 22 by a pulley/belt assembly or gear box 26. The drive head 20 typically includes a stuffing box (not shown), a clamp 28, and a polished rod 29. The stuffing box is used to seal the connection of drive head 20 to drive shaft 30, and clamp 28 and polished rod 29 are used to transmit the rotation from the drive head 22 to the drive shaft 30.
Downhole, progressive cavity pump 40 installs below the wellhead 12 at a substantial depth (e.g., about 2000 m) in the wellbore. Typically, pump 40 has a single helical-shaped rotor 42 that turns inside a double helical elastomer-lined stator 44. During operation, the stator 44 attached to production tubing string 14 remains stationary, and surface drive 20 coupled to rotor 42 by drive string 30 cause rotor 42 to turn eccentrically in stator 44. As a result, a series of sealed cavities form between stator 42 and rotor 44 and progress from the inlet end to the discharge end of pump 40, which produces a non-pulsating positive displacement flow.
Because pump 40 is located at the bottom of the wellbore, which may be several thousand feet deep, pumping oil to the surface requires very high pressure. The drive shaft 30 coupled to the rotor 42 is typically a steel stem having a diameter of approximately 1″ and a length sufficient for the required operations. During pumping, shaft 30 may be wound torsionally several dozen times so that shaft 30 accumulates a substantial amount of energy. In addition, the height of the petroleum column above pump 40 can produce hydraulic energy on drive shaft 30 while pump 40 is producing. This hydraulic energy increases the energy of the twisted shaft 30 because it causes pump 40 to operate as a hydraulic motor, rotating in the same direction as the twisting of drive shaft 30.
The sum total of all the energy accumulated on drive shaft 30 will return to the wellhead when operations are suspended for any reason, either due to normal shutdown for maintenance or due to lack of electrical power. A braking system (not shown) in drive 20 is responsible for blocking and/or controlling the reverse speed resulting from suspension of the operations. When pumping is stopped, for example, the braking system is activated to block and/or allow reverse speed control and dissipate all of the energy accumulated on the shaft 30. Otherwise, the pulleys or gears of box 26 would disintegrate or become damaged due to the centrifugal force generated by the high rotation that would occur without the braking system. Current braking systems have a brake screw 23 that can be operated directly by an operator. Turning the brake screw 23 can apply or release an internal brake shoe that, in turn, presses on a rotating drum, causing a braking effect to shaft 30.